py7zr: O(n^2) algorithmic complexity DoS in PackInfo._read()

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-55206: 0.27% estimated exploitation activity in the next 30 days; percentile 18.83 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary PackInfo.read() uses an O(n^2) cumulative sum pattern where numstreams is read directly from the archive header. A crafted .7z archive with a large numstreams value causes excessive CPU consumption during SevenZipFile.init() — no extraction is needed. A 50 KB archive takes ~7 seconds of CPU time. Details The vulnerable code is in PackInfo.read() (archiveinfo.py): self.packpositions = [sum(self.packsizes[:i]) for i in range(self.numstreams + 1)] numstreams is parsed from the archive header via readuint64() and is attacker-controlled. Each sum(self.packsizes[:i]) re-sums from the beginning, producing O(n^2) total work. This runs during header parsing in SevenZipFile.init(), before any extraction. Suggested fix — replace with O(n) cumulative sum: from itertools import accumulate self.packpositions = [0] + list(accumulate(self.packsizes)) PoC Tested on py7zr 1.1.0, Python 3.12.3, Linux x8664. Results: n= 1000 size= 1042 bytes time=0.004s n= 5000 size= 5042 bytes time=0.071s n=…

Published by the source

19 Jun 2026, 21:16 UTC

Last modified by the source

13 Jul 2026, 16:43 UTC

Source last checked

26 Oct 2026, 13:46 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-h4gh-22qq-72r7

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-h4gh-22qq-72r7

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# Patch Next advisory brief: CVE-2026-55206

Advisory: py7zr: O(n^2) algorithmic complexity DoS in PackInfo._read()
IDs: CVE-2026-55206, GHSA-h4gh-22qq-72r7
Record: https://patchnext.com/advisories/cve-2026-55206 (revision 3, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: py7zr 1.1.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI py7zr: affected <= 1.1.2; fixed in 1.1.3

## Suggested inspection (read-only)
- `python -m pip show py7zr`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on py7zr.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-h4gh-22qq-72r7 (OSV record GHSA-h4gh-22qq-72r7 lists affected ranges: PyPI py7zr >=0 <1.1.3 (fixed))
- Verified: https://github.com/advisories/GHSA-h4gh-22qq-72r7 (GitHub-reviewed advisory GHSA-h4gh-22qq-72r7 lists affected ranges: PyPI py7zr >=0 <=1.1.2)

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